The application relates to secure communication over a passive optical network.
Quantum key distribution (QKD) provides a secure communication between two legitimate users: Alice (a sender) and Bob (a receiver). Its security is proved to be unconditional based on the fundamental laws of physics. It is natural to extend QKD from the point-to-point setting to a network setting. There can be many schemes to implement QKD in a network. One appealing scheme is to integrate QKD in a passive-optical-network (PON).
Recently, PON has attracted much interest as an inexpensive and efficient solution to the “last mile link”, or access network. FIG. 1 shows an exemplary PON. An Optical Line Terminal (OLT) 10 broadcasts a data stream over a power splitter 12 to several Optical Network Units (ONUs) 14. The data stream is passively routed to ONUs 14 using techniques such as the Time Domain Multiplexing (TDM), the Wavelength Domain Multiplexing (WDM), and the Orthogonal Frequency Domain Multiplexing (OFDM).
Most current quantum crypto systems are based on point-to-point scheme. It is important to find a solution that can extend the quantum cryptography technology into a network setting. Passive optical network (PON) provides an ideal network platform that has the potential to supply quantum encryption. It has attracted much academic and industrial interest to integrate the quantum cryptography technology with PON. This integration will substantially improve the accessibility of quantum encryption, which can provide the highest communication security for real-life applications.
In TDM, the OLT assigns each ONU a specific time slot. During each time slot, the data broadcasted by the ONU is dedicated for the allocated user. The data stream broadcasted by the OLT is passively splitted by an optical splitter. As a result, each ONU sees the entire stream. The ONU picks up the correct data section that is assigned to the user.
FIG. 2A shows an exemplary TDM PON with an optical beam splitter (OBS) 16. One issue with the TDM-PON is that each user's bandwidth will decrease as the number of users in the PON increases. An alternative implementation is to use the wavelength-division multiplexing (WDM) technique. FIG. 2B shows an exemplary WDM-PON where each user is assigned a unique wavelength. An OLT 20, which includes an array of transceivers at different wavelengths multiplexed with WDM 22, communicates with other WDMs 24 and with multiple transceivers at the ONU side. Each transmitter has a dedicated ONU 14. The signals from all the transmitters are combined into the same channel with a WDM 24, and routed to different users with another WDM 22.
Integrating quantum encryption with PON has been done by distributing the secure keys in a down-stream (i.e., from server to users), uni-directional (i.e., the server generates a signal, then sends it to a user), and time-domain multiplexing (TDM) fashion. Down-stream key distribution requires that each user possesses a pair of single photon detectors (SPDs) which are expensive. Moreover, in such a scheme, upgrading the network (i.e., increasing the bandwidth) requires that each user has to replace his/her pair of SPDs. Therefore, the down-stream key distribution is expensive in initial deployment and future upgrading. Uni-directional key distribution requires active optical alignment compensation for the phase drift and the polarization dispersion. This active compensation will reduce the overall duty cycle and increase the operating complexity.
There are various QKD protocols, and among these protocols, BB84, B92, and E91 are proved to be unconditionally secure. However, implementing B92 protocol requires accurate intensity monitoring, while E91 protocol requires an entanglement source. Therefore, among these three protocols, BB84 is most widely implemented and is most mature. The BB84 protocol can be used with double Mach-Zehnder interferometer (MZI), Faraday-Michelson interferometer (FMI), and Plug-and-Play (PnP) structures. Other BB84 implementation structures (such as free-space structure, fiber-based polarization coding structure, among others) can be used as well.
FIG. 2C shows one embodiment with the PnP structure where the polarization-drift in the channel is compensated by the bi-directional structure and the Faraday rotation. The phase drift in local MZI is also compensated by the bi-directional structure. FIG. 2C shows a schematic of PnP QKD with Single Photon Detector (SPD) 40, 42; Phase Modulator (PM) 44,46; Faraday Mirror (FM) 48; Circulator (C) 50; Beam Splitter (BS) 52; Polarizing Beam Splitter (PBS) 54 and a delay line 56. In network applications, a significant advantage for PnP QKD is that both the laser source 60 and the detectors are in Bob's side, making Alice's apparatus very simple. In network environment, especially PON which is in server-user scheme, PnP will allow multiple users (Alice) to share a server (Bob).
To accommodate PnP implementation, WDM has significant advantage by minimizing channel loss. This is because, in TDM, the channel loss will increase as the number of users increases. However, the output intensity at each user's side cannot increase much due to the security requirement. This can substantially reduce the efficiency of QKD.